As you probably know, Pathfinder Beginner Box has no rules concerning encumbrance or weight. You may be comfortable just hand waving encumbrance issues, but if you want something a little more concrete I suggest the following:
First, if you check the BB character sheet, you will note that the equipment section has 30 entries (or “slots,” if you will). Simply treat these 30 slots as a hard limit on how much stuff characters can carry, including armor, shield, weapons, equipment and treasure (to include gems, jewelry, artwork, potions, scrolls, wands and other magic items). This also includes items that occupy one of the 13 item slots specified on pg. 48 of the Game Masters Guide. So, characters have a maximum of 30 inventory ‘slots.’
For the simplest application of this, simply treat everything as a separate item, even really small things like pitons or darts. For example, if a character buys 5 darts, those darts occupy 5 different inventory slots. This may seem a bit ridiculous at first glance, but consider that while one slot holds a dart, another may hold a suit of plate mail. We assume that weight carried averages out across all 30 slots.
In addition, cross out the GP/SP/CP boxes at the top of the equipment section and have all coins recorded within the equipment slots, 100 coins per slot. This enforces an encumbrance limit on treasure and will force players to make hard choices about what to take home with them (perhaps even ditching vital equipment to make more room for treasure).
If you want to be a bit more forgiving, allow small items to be combined in sacks or pouches to make a single item: 10 pitons or darts, 5 torches, 100 coins, etc. For the sake of simplicity, all items combined in a sack should be the same (so, no mixing pitons and coins, for example). Likewise, keep treasure items such as gems and jewelry as separate inventory items (it’s a fantasy game, so assume really big, gaudy gems and jewels 🙂 ).
GM’s are free to declare that some items, based on common sense, simply can’t be carried by a single character no matter how many inventory slots they have open. For example, even though a horse is an individual item in the equipment charts, it doesn’t make sense that a character could carry a horse around with them (record the horse on the character sheet under Notes instead of Equipment); likewise a large chest filled with treasure, or a throne made of gold.
A horse used as a pack animal also has 30 inventory slots. A riding horse can carry a character and, say, another 10 slots worth of equipment.
Finally, if you feel that it’s not realistic to have every character carry the same amount of gear, regardless of their Strength scores, then try something like this: Each character has 10 + Strength inventory slots, hard capped at 30 (i.e. a character with 18 Strength has 28 inventory slots).